In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) resonator. The BAW resonator includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack. One type of BAW resonator comprises a piezoelectric film for the piezoelectric material. These resonators are often referred to as Film Bulk Acoustic Resonators (FBAR).
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns. FBARs beneficially provide a comparatively compact alternative to certain known resonators.
FBARs may comprise a membrane (also referred to as the acoustic stack) disposed over air. Often, such a structure comprises the membrane suspended over a cavity provided in a substrate over which the membrane is suspended. Other FBARs may comprise the membrane formed over an acoustic mirror formed in the substrate. Regardless of whether the membrane is formed over air or over an acoustic mirror, the membrane comprises a piezoelectric layer disposed over a first electrode, and a second electrode disposed over the piezoelectric layer.
The piezoelectric layer comprises a crystalline structure and a polarization axis. Piezoelectric materials either compress or expand upon application of a voltage. By convention, a piezoelectric material that compresses when a voltage of a certain polarity is applied is referred to as compression-positive (CP) material, whereas a piezoelectric material that expands upon application of the voltage is referred to as a compression-negative (CN) material. The polarization axis of CP piezoelectric materials is antiparallel to the polarization axis CN material.
An FBAR is a polarity-dependent device as a result of polarity dependence of the piezoelectric material that constitutes part of the FBAR. A voltage of a given polarity applied between the electrodes of the FBAR will cause the thickness of the FBAR to change in a first direction, whereas the same voltage of the opposite polarity will cause the thickness of the FBAR to change in a second direction, opposite the first direction. (The thickness of the FBAR is the dimension of the FBAR between the electrodes.) For example, a voltage of the given polarity will cause the thickness of the FBAR to increase whereas a voltage of the opposite polarity will cause the FBAR to decrease. Similarly, a mechanical stress applied to the FBAR that causes the thickness of the FBAR to change in a first direction will generate a voltage of the given polarity between the electrodes of the FBAR, whereas a mechanical stress that causes the thickness of the FBAR to change in a second direction, opposite the first direction, will generate a voltage of the opposite polarity between the electrodes of the FBAR. As such, a mechanical stress applied to the FBAR that causes the thickness of the FBAR to increase will generate a voltage of the given polarity, whereas a mechanical stress that causes the thickness of the FBAR to decrease will generate a voltage of the opposite polarity.
The piezoelectric layer of an FBAR is often grown over a first electrode and beneath a second electrode. The orientation of the C-axis can be governed by the first layer formed over the first electrode. For example, in growing aluminum nitride (AlN) with a CP film orientation, the formation of a native oxide layer over the first electrode (e.g., Mo) is believed to cause the first layer of the piezoelectric crystal to be Al. Ultimately, the crystalline orientation of the AlN formed results in piezoelectric film's having CP orientation and its attendant properties. Growth of CN piezoelectric layers (e.g., AlN) by known methods has proven to be more difficult. It is believed that nitrogen and oxygen may be adsorbed at the surface of the first electrode, with the forming of a layer of Al over this adsorbed material. As such, rather than forming the desired CN piezoelectric layer, CP piezoelectric material is formed.
In certain applications, it is desirable to be able to select the orientation of the piezoelectric material, and to fabricate both CP piezoelectric material and CN piezoelectric material on the same structure. For example, in certain applications it is useful to provide a single-ended input to a differential output. One known resonator structure having a differential output comprises coupled mode resonators. Filters based on coupled mode acoustic resonators are often referred to as coupled resonator filters (CRFs). CRFs have been investigated and implemented to provide improved passband and isolation of the transmit band and receive band of duplexers, for example. One topology for CRFs comprises an upper FBAR and a lower FBAR. The two electrodes of one of the FBARs comprise the differential outputs, and one of the inputs to the lower resonator provides the single-ended input. The second electrode provides the ground for the device. However, while the stacked-FBAR CRF shows promise from the perspective of improved performance and reduced area or footprint due to its vertical nature, in order to attain this structure, the orientation of the compression axes (C-axes) of individual piezoelectric materials must be tailored to the application. For example, it may be useful to have one piezoelectric layer with its C-axis (e.g., CN) in one direction, and the second piezoelectric layer to have its crystalline orientation anti-parallel (e.g., CP) to the C-axis of the first piezoelectric layer. Unfortunately, and as alluded to above, using known methods of fabricating piezoelectric layers, it is difficult to select the orientation of the piezoelectric crystal during fabrication, and especially on the same wafer.
What is needed, therefore, is a method of fabricating piezoelectric materials that overcomes at least the known shortcomings described above.